Aage R. Møller

Responses of neurons in the inferior colliculus of the rat to AM and FM tones

The responses of units in the inferior colliculus of the urethane-anaesthetized rat were recorded extracellularly. They responded to sinusoidal AM and FM tones with a modulation of their spike discharge usually at the same, or occasionally at twice, the modulation rate of the stimulus. The modulation depth of the response initially increased with the modulation depth of the stimulus, but usually saturated or decreased at higher stimulus depths. The units showed a bandpass tuning to stimulus modulation rate which was independent of modulation depth and, in all cases, the most effective modulation rate was below 120 Hz. The modulated response to temporally varying stimuli could not be predicted from the pure tone discharge patterns or, in some cases, the units mean firing rate to modulated tones; temporally varying stimuli gave temporally varying responses. When compared with the data available from units at other levels in the auditory system, the results indicate a trend in which units at successively higher levels in the pathway respond most effectively to progressively lower rates of modulation.

Pathophysiology of tinnitus

Tinnitus is not a single entity but a rather diverse group of disorders. Despite symptoms that indicate the ear is the site of the pathology, there is strong evidence that most forms of severe tinnitus are caused by functional changes in the central nervous system. The changes are induced through expression of neural plasticity, some of which may have been caused initially by abnormalities in the ear or the auditory nerve. The involvement of the nonclassical ascending auditory pathway with its subcortical connections to limbic structures (the amygdala) may explain some of the symptoms of some forms of tinnitus including hyperacusis and affective disorders, such as phonophobia and depression, which often accompany severe tinnitus.

An Experimental Study of the Acoustic Impedance of the Middle Ear and Its Transmission Properties

The acoustic impedance at the eardrum and the cochlear microphonic potential at constant sound pressure level at the eardrum were measured in anesthetized cats and rabbits. It was found that the inverse of the impedance (admittance) and the cochlear microphonic potential at constant sound pressure are proportional over a large frequency range. In addition, the effects of opening of the middle-ear cavities and of variation of the air pressure in the cavity, as well as of activity of the middle-ear muscles were studied. In further experiments, the impedance of the eardrum itself and of the middle ear with the cochlea disconnected was measured.

Transcranial magnetic stimulation for tinnitus: influence of tinnitus duration on stimulation parameter choice and maximal tinnitus suppression.

Objective: Tinnitus is a distressing symptom for which few treatments exist. It leads to an important decrease in quality of life in 2 to 3% of the population. Tinnitus is considered a phantom sound, the result of cortical reorganization. Transcranial magnetic stimulation (TMS) is a noninvasive method to modulate cortical reorganization and has been shown to be able to influence tinnitus perception. Study Design: Retrospective analysis. Setting: Tertiary referral center. Patients: The effect of TMS of the contralateral auditory cortex in 114 patients with unilateral tinnitus is investigated as one of the selection criteria used for surgical implantation of electrodes on the auditory cortex. Intervention: TMS is performed at 90% of motor threshold at 1, 3, 5, 10, and 20 Hz, with each stimulation session consisting of 200 pulses. Results were classified as no effect (0-19% improvement), partial effect (20-79% improvement), and good effect (80-100 suppression). Main Outcome Measures: TMS had a good effect in 25% of the patients studied, partial effect in 28% patients, and no effect in 47%. Results: TMS at 200 pulses is capable of tinnitus suppression for seconds only. The results were influenced by tinnitus duration: the longer the tinnitus exists, the lower the stimulation frequency that yields maximal tinnitus suppression (p < 0.001). The maximal amount of tinnitus suppression decreases in time (p < 0.01), resulting in a 2% decrease of potential tinnitus suppression per year. Conclusion: TMS of the auditory cortex is capable of modifying tinnitus perception for a very short time. The maximal amount of suppression and best stimulation frequency depends on the tinnitus duration.

Some forms of tinnitus may involve the extralemniscal auditory pathway

It has previously been shown that the click‐evoked responses recorded from the intracranial portion of the eighth nerve in patients with incapacitating tinnitus are not abnormal, nor is the latency of peak III of the click‐evoked brainstem auditory‐evoked potentials significantly altered; however, the latency of peak V is slightly (but significantly) shortened in comparison to that of patients with the same degree of hearing loss but no tinnitus. In this study the hypothesis that the extralemniscal auditory system is involved in the generation of tinnitus is tested. We made use of the fact that neurons of the extralemniscal auditory system also receive input from the somatosensory system, and that stimulation of the somatosensory system can influence the processing of auditory information in the extralemniscal system. In 4 of 26 patients with mild‐to‐severe tinnitus whose median nerve was stimulated electrically, the tinnitus increased noticeably during stimulation, in 6 the intensity of the tinnitus decreased noticeably, and in the remaining 16 there was no noticeable change in the tinnitus. In some of the patients the character of the tinnitus changed in a complex way. There were no significant differences in hearing thresholds in these three groups of patients. Electrical stimulation of the median nerve in 12 individuals with normal hearing who did not have tinnitus either had no effect on the loudness of sounds or it caused a slight increase in the loudness.

Intracranially recorded responses from the human auditory nerve: New insights into the origin of brain stem evoked potentials (BSEPs)

Auditory evoked potentials were recorded intracranially from the 8th nerve during neurosurgical procedures. The potentials had a large negative peak that occurred 3.0--3.7 msec after the onset of the stimulus (2 000 Hz tone bursts). When these potentials were compared with the scalp recorded brain stem evoked potentials (BSEPs) the intracranial response was found to match the latencies of the P2N3 complex of the BSEP. The results are interpreted as showing that the neural generator of the second peak of the BSEP is the intracranial portion of the auditory nerve and not, as was earlier assumed, the cochlear nucleus.

Disabling positional vertigo

We have identified a group of patients with vestibular disorders whose symptoms are not consistent with the commonly recognized syndromes such as Menieres disease, benign paroxysmal positional vertigo, and vestibular neuronitis. These patients have a constant positional vertigo and are often nauseated to an extent that makes them disabled. Their symptoms do not respond to conventional medical treatment or habituating therapy. We have found specific clinical-pathological signs in these patients that indicate that the vestibular nerve is compressed intracranially by blood vessels. Treatment of nine such patients by microvascular decompression of the eighth nerve brought total relief of symptoms in eight patients and improvement in one. We suggest that this syndrome be named disabling positional vertigo.

Evoked potentials from the inferior colliculus in man

Sound-evoked potentials were recorded from the inferior colliculus in man when it was exposed during surgery. The earliest response to controlateral stimulation with 2000 Hz tone bursts at 90 dB was a positive deflection with a latency of about 6.5 msec that was followed by a slow, negative deflection that lasted about 5 msec. It is supposed that this surface-positive peak originates in the lateral lemniscus. Its latency matched that of the fifth vertex-positive wave (V) of the BSEP recorded from the scalp. Superimposed on the slow potential were several peaks. These peaks emerged clearly after the slow components were removed by filtering. The peaks then were shown to have latencies that matched the latencies of peaks VI, VII and VIII of the scalp-recorded BSEP.

Stimulus properties influencing the responses of inferior colliculus neurons to amplitude-modulated sounds

The temporal pattern of the responses of neurons in the inferior colliculus of the anesthetized rat were studied using continuous tone or noise carrier signals, amplitude modulated by pseudorandom noise. Period histograms of the responses, cross-correlated with the pseudorandom noise, gave an estimate of the units impulse responses to modulation. The amplitude-modulation rate transfer function (MTF) was obtained by Fourier transforming the correlograms. At sound levels within approximately 15 dB of the unit threshold, the MTFs were near lowpass functions between 6 and 200 Hz but became more bandpass-like as the intensity was increased. There was a steep decline in the response to modulation at modulation frequencies above 200 Hz for all stimulus intensities. For the bandpass-type MTFs the greatest modulation of the discharge pattern occurred at modulation frequencies between 10 and 200 Hz with a maximum in the distribution of MTF peak values between 100 and 120 Hz. There was no consistent relationship with characteristic frequency of either the position of the MTF peak or the high-frequency cutoff of the MTF. The cross-correlograms obtained at high stimulus intensities (30-60 dB above threshold) often showed a negative peak, representing a decrease in the probability of firing in response to intensity increments in the stimulus, and denoting a nonmonotonic rate-intensity function. The MTFs for units responding to amplitude-modulated broadband noise were often flatter in the low frequency region than those generated with tone carriers at corresponding intensities. For some units addition of a broadband noise background to the modulated tone changed the response characteristic of the MTF from bandpass to lowpass and shifted the MTF peak to a lower modulation frequency. The results demonstrate that although neurons in the inferior colliculus are selectively sensitive to the modulation frequency of dynamic stimuli, the response characteristics are not invariant, but instead are closely dependent on the conditions under which the modulation is presented.

Microvascular decompression in hemifacial spasm: Intraoperative electrophysiological observations

Facial muscle responses in patients with hemifacial spasm undergoing microvascular decompression operations were recorded. Two peripheral branches of the facial nerve were stimulated and the electrical responses of muscles innervated by these branches were studied to see how the lateral spread of activity that is known to be present in these patients was affected by decompressing the facial nerve. In some of the patients the hemifacial spasm ceased when the dura mater was opened, in some it ceased when the arachnoid was opened, and in others the spasm persisted until the offending vessel was dissected away from the nerve. The lateral spread of activity elicited by antidromic stimulation of a branch of the facial nerve was less affected by opening of the dura mater or arachnoid: it usually persisted until the blood vessel that had been compressing the facial nerve was removed and reappeared when the vessel that had been compressing the facial nerve was allowed to slip back onto the nerve. This seems to indicate that microvascular decompression of the facial nerve is effective in alleviating hemifacial spasm because it removes the actual cause of the disorder rather than simply causing local injury to the nerve as a result of the procedure.